Hight fuel pump noise

What fuel are you using?   If you are using Automotive fuel in warm weather, then the fuel may be beginning to vaporize and causing the pumps to cavitate. They are louder when cavitating, and have a higher pitch. As you climb the fuel will vaporize easier due to the lower ambient pressure, and its also possible your fuel is getting warmer as it recirculates through the engine compartment, which can also make the issue worse. If this is what is happening, it can lead to vapor lock.

Here in the US we have different blend fuels for summer and winter, but I don't know if that's the case where you live.  The winter fuels here are much more prone to vaporization, so a lot of folks run a 50/50 mix of Automotive fuel and Avgas in the warmer winter months (spring), until the refineries switch over to Summer Blend automotive fuel.     

What pumps are you using, and in what aircraft?

    We have solved the problem by installing a larger prefilter with a larger filtering surface. The Installation manual does not mention the maximum drop in pressure that can exist in the filter or the minimum volume or surface area of ​​the filter. I had an Andair FX375-MK installed. With the new filter with more than double the filtering capacity, the pumps have stopped making noise.

Thanks 

Ernesto                    

Sounds like your pre-filter was not allowing enough fuel to flow and this can cause the cavitation and extra odd noise. Better to have a slightly larger filter than one too small for your inlet fuel flow.

Yes, but In the Rotax Installation Manual there is no specification of the filter to be installed, except for the size of the mesh, I think that the manual should have some specification regarding this filter.

Sincerely

Ernesto